The seeds pictured below fall around my neighbourhood every Autumn (Southern Hemisphere - New Zealand). The seeds are blown over quite a vast area and I haven't been able to identify the tree(s) they're coming from (possibly on private property where I can't get near).
I suspect they're either sycamore or maple seeds but Google Images show them to both look very similar. I was hoping someone might be able to give me a more definitive suggestion.
Click photo for full size
A sycamore is a common name for an Acer, (except in America), Acer pseudoplatanus to be precise. Given that the sycamore tends to produce an inordinate number of seeds compared to other Acers, it's most likely that.
UPDATE: You can certainly try growing one or two, but in my experience, if your garden is covered in these, you will inevitably miss some and they will inevitably germinate... and then you'll know for sure. You could have another wander round the neighbourhood looking specifically for an Acer pseudoplatanus, but these seeds are windblown, and can come from a fair distance away when there's a strong wind.
Only one Acer can be classed as a sycamore, and that is Acer pseudoplatanus.
Here is a picture of the seeds of A. pseudoplatanus:
Very similar to your picture, but with a slightly narrower blade, and slightly longer seed. This may be because of locality, or variation within species, but there are other maple species that this could possibly be. I'm not in New Zealand, so I won't know which species to expect there, but if it is A. pseudoplatanus, look for a tree something like:
with bark something like:
and leaves something like:
